We compared the development of the Lyme disease spirochete, Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt and Brenner, in subadult rabbit-feeding Ixodes dentatus Marx with that in mouse-feeding I. dammini Spielman, Clifford, Piesman and Corwin. Rabbits were infected with spirochetes by the bites of I. dammini that had been infected naturally in a zoönotic site. Larval ticks of both species were permitted to engorge simultaneously on each of these infected hosts. Spirochetes were present in the guts of about half of the resulting nymphal I. dentatus and most of the I. dammini that developed. An experimentally infected nymphal I. dentatus, in turn, infected a rabbit. Because I. dentatus feeds solely on rabbits, and these hosts may be extraordinarily abundant in nature, this tick provides potential for a hidden enzoötic cycle of natural Lyme disease transmission.